Distance measuring device and distance image synthesizing method

1. A distance measuring device, comprising: a light emitter which emits light from a light source to a target object; a light receiver which receives reflected light in a group of pixels two-dimensionally disposed, the reflected light being generated through reflection of the light emitted from the light emitter on the target object; a synthesizer which generates a synthesized signal by synthesizing a plurality of pixel signals read from pixels exposed at different exposure intervals among the group of pixels; a distance arithmetic operator which calculates a value of distance to the target object based on the synthesized signal and a difference in time between emission and reception of the light; and a movement detector which detects a movement of the target object, wherein the synthesizer generates the synthesized signal by first synthesis processing when a pixel signal of a pixel exposed at a first exposure interval has a signal level higher than a predetermined saturation level, and generates the synthesized signal by second synthesis processing when the pixel signal of the pixel exposed at the first exposure interval has a signal level lower than the predetermined saturation level, and in the first synthesis processing and the second synthesis processing, the synthesizer generates the synthesized signal using the pixels in the previous frame when the movement is smaller than a threshold, and generates the synthesized signal using the pixels in the current frame when the movement is not smaller than the threshold.

2. The distance measuring device according to claim 1 , wherein in the first synthesis processing, the synthesizer generates the synthesized signal using a pixel signal of a pixel exposed at a second exposure interval shorter than the first exposure interval among the plurality of pixel signals.

3. The distance measuring device according to claim 2 , wherein in the second synthesis processing, the synthesizer generates the synthesized signal by adding the pixel signal of the pixel exposed at the first exposure interval to the pixel signal of the pixel exposed at the second exposure interval among the plurality of pixel signals.

4. The distance measuring device according to claim 3 , wherein in the first synthesis processing, the synthesizer generates a synthesized signal through interpolation processing using the pixel signal of the pixel exposed at the second exposure interval, and the synthesized signal is expected to be obtained in the second synthesis processing when the signal level is lower than the predetermined saturation level.

5. The distance measuring device according to claim 2 , wherein pre-synthesized pixels exposed at the first exposure interval and pre-synthesized pixels exposed at the second exposure interval are disposed symmetrical about a center of gravity of post-synthesized pixels.

6. The distance measuring device according to claim 5 , wherein the group of pixels outputs first pixel signals read from pixels in a first line which are exposed at the first exposure interval and second pixel signals read from pixels in a second line different from the first line which are exposed at the second exposure interval, and the first line and the second line are alternately disposed in one frame.

7. The distance measuring device according to claim 6 , wherein the group of pixels is disposed such that the pixels in the first line are shifted in a horizontal direction from the pixels in the second line by a half of a pixel.

8. The distance measuring device according to claim 7 , wherein a ratio of the first exposure interval to the second exposure interval is 4:1.

9. The distance measuring device according to claim 6 , wherein the light receiver alternately generates a frame having the first line as a leading line and a frame having the second line as a leading line.

10. The distance measuring device according to claim 1 , wherein the synthesizer includes a background light remover which removes a pixel signal indicating background light from a pixel signal indicating the reflected light, and generates the synthesized signal using a pixel signal indicating the reflected light after removal of the pixel signal indicating the background light.

11. The distance measuring device according to claim 10 , wherein the synthesizer includes a gain corrector which performs gain correction on the synthesized signal obtained by the first synthesis processing based on a ratio of the exposure intervals such that a signal level of the synthesized signal obtained by the first synthesis processing is adjusted to a signal level of the synthesized signal obtained by the second synthesis processing, and the distance arithmetic operator calculates the value of distance using the synthesized signal after gain correction.

12. A distance measuring device, comprising: a light emitter which emits light from a light source to a target object; a light receiver which receives reflected light in a group of pixels two-dimensionally disposed, the reflected light being generated through reflection of the light emitted from the light emitter on the target object; a synthesizer which generates a synthesized signal by synthesizing a plurality of pixel signals read from pixels exposed at different exposure intervals among the group of pixels; a distance arithmetic operator which calculates a value of distance to the target object based on the synthesized signal and a difference in time between emission and reception of the light; a frame memory which records all of the pixel signals in an immediately preceding frame obtained in the light receiver; a movement detector which detects a movement of the target object; a first interpolated pixel generator which generates first pixel signals for all the pixels from the first pixel signals; and a second interpolated pixel generator which generates second pixel signals for all the pixels from the second pixel signals, wherein the synthesizer generates the synthesized signal by first synthesis processing when a pixel signal of a pixel exposed at a first exposure interval has a signal level higher than a predetermined saturation level, and generates the synthesized signal by second synthesis processing when the pixel signal of the pixel exposed at the first exposure interval has a signal level lower than the predetermined saturation level, pre-synthesized pixels exposed at the first exposure interval and pre-synthesized pixels exposed at the second exposure interval are disposed symmetrical about a center of gravity of post-synthesized pixels, the group of pixels outputs first pixel signals read from pixels in a first line which are exposed at the first exposure interval and second pixel signals read from pixels in a second line different from the first line which are exposed at the second exposure interval, the first line and the second line are alternately disposed in one frame, and the light receiver alternately generates a frame having the first line as a leading line and a frame having the second line as a leading line, and in the second synthesis processing, the synthesizer synthesizes a pixel signal of a current pixel with a pixel signal of a co-located pixel in an immediately preceding frame when the movement detector determines that the target object is in a static state, and synthesizes the first pixel signal generated by the first interpolated pixel generator with the second pixel signal generated by the second interpolated pixel generator when the movement detector determines that the target object is in a dynamic state.

13. The distance measuring device according to claim 12 , wherein from the pixel signals obtained in the light receiver, a first interpolated image composed of the first pixel signal is generated by the first interpolated pixel generator, and a second interpolated image composed of the second pixel signal is generated by the second interpolated pixel generator, from the pixel signals obtained in the immediately preceding frame recorded in the frame memory, a third interpolated image composed of the first pixel signal is generated by the first interpolated pixel generator, and a fourth interpolated image composed of the second pixel signal is generated by the second interpolated pixel generator, the movement detector compares the first interpolated image to the third interpolated image and the second interpolated image to the fourth interpolated image to detect a movement of the target object, and in the first synthesis processing, the synthesizer generates the synthesized signal by the first synthesis processing using the fourth interpolated image when the movement is smaller than a threshold, and generates the synthesized signal by the first synthesis processing using the second interpolated image when the movement is not smaller than the threshold.